Starting of an AC induction motor produces inrush current.
Starters of AC induction motor changing characteristics of electric current supplied to an AC induction motor are widely used to produce desired starting process and to make inrush current lower.
In wildly used in modern industry starters: star-delta starter and auto-transformer starter, changes of voltage supplied to the motor fulfill step-by-step. Because of small number of steps and changing of steps in moments, which are far from optimal, the quality of start process is insufficient. Besides this, auto-transformer is used almost all time of start process. This fact influences on size of the auto-transformer.
Advantage of star-delta starter is that it uses electromagnetic system of the started motor by addition only two contactors and timer without outer electromagnetic system for starting. It is also cheapest of starters. It is easy to regulate it.
Star-delta starter permits to increase power consumption of the motor three times when star configuration of motor windings is changed to delta configuration. It is clear from this fact that an AC induction motor can receive on step of delta configuration of the windings two times greater addition of power than the addition of power on start step of star configuration of motor windings. That is the reason that current and duration of delta step of starting are greater than on star step.
In (PCT WO 00/77919) is made an attempt to start and control AC motor with better characteristics by periodically changing time of star period of motor starting because of aging of motor.
Addition of only one step of motor power changing between powers of star and delta configuration of the windings will permit to do more equivalent step additions of motor power and to receive equal quality during start period. Sometimes to achieve breakaway torque enough to supply to the motor less power than the motor receives on step of star configuration. With increasing number of steps it is possible to improve quality of motor starting, to make smoother motor starting, especially in difficult cases.
In (JP 200217117) is made an attempt to increase number of steps in delta configuration by introducing in each phase non-linear element parallel to power contact of contactor, producing delta configuration. This element permits two schemes in delta configuration: motor winding connected in series with non-linear element and motor winding without non-linear element. However, in star configuration this element increases current supplied to the motor because it is parallel to motor winding.
It is important to choose simple and cheap elements with constant characteristics, addition of which into structure of star-delta starter transform it into star-delta multi-level starter.
It is known control a system (U.S. Pat. No. 7,330,000), which is directed to controlling the level of a supply voltage applied to a load. It comprises circuitry, including at least one transformer having a primary winding being connectable across the supply voltage and a secondary winding being connected in series with the load. Further, a switching assembly is provided and operatively structured to be disposed in a first position serving to connect the primary winding across the supply voltage, thereby providing a voltage having an opposite polarity being introduced across the secondary winding of the transformer. Moreover, the switching assembly, being disposed in a second position, serves to disconnect the primary winding from across the supply voltage and to shunt the primary winding so that voltage across the secondary winding is zero.
Control circuit of star-delta starter changes open and close position of contactors, which define motor windings configuration, through preset time interval introduced into timer. However, with increasing number of steps of addition of power to the motor it is increased difficulty of selecting of time intervals of the steps or moments of steps changing. It is preferable, if control circuits will do it automatically.
It is also needed sometimes that a stop process of AC induction motor will be within definite characteristics, for example, smooth stopping. In soft starters smooth stopping is achieved by gradually increasing firing angle of thyristors (U.S. Pat. Nos. 4,052,648, 5,008,608).
It must be understood, that effective starting and stopping of an AC induction motor may be achieved, if the improvement will concern two aspects:                increasing number of stepped changing of the motor power consumption,        starting new step in the moment, when transition conditions of previous step are near to be finished.        
Therefore, it is a primary object and feature of present invention to provide an improved star-delta starter as star-delta multi-level starter for controlling the starting and stopping of an AC induction motor.
It is a further object and feature of present invention to provide an improved control circuit for controlling the starting and stopping of an AC induction motor in effective manner.
It is still a primary object and feature of present invention to provide an improved star-delta starter for controlling the starting and stopping of an AC induction motor, which utilizes traditional and less expensive components.